Separation of nitrogen from methane



Feb.- 18, 1958 B. E. EAKIN ET AL 2,823,523

SEPARATION OF NITROGEN FROM METHANE Filed Mar'ch 26, 1956 EXPANSION TURBINE d6 MED- PRESSURE SEPARATOR LIQUID FEED SUB- COOLER COLUMN FEED GAS CON- DENSER LOW PRESSURE FEED FEED PR D COMPRESSOR REBOILER BYPASS 42 HIGH PRESSURE T PRODUCT VAPORIZER WATE '10 43 I In M22250? 1" fierzl E. Ea/Jz'zz Rex IT. EZZzny-zfozz Til,

5. 2362 SEPARATIQN or NITROGEN FROM MET-HANE l lertE- Eakim; Chicago, Re'x ,T. Ellington, Evanston,

.IlL, assig ngrs to fl he Ipstitute, of. Gas Technology, Chi fcago, 111., a col pol'ation of Illinois Application March 26, 1956, Serial No. 573,865 aclaim tomb-175.5

This invention relates to an improved process for separating nitrogen from methane, or natural gas, which consists essentially of methane The use of natural gas inthe-United States has expanded enormously in recent years and the demand continues toincrease. As the reserves of relativelypure natural gas are depleted, natural gas of higher nitrogen content becomes a more valuable resource. It is desirable, however, to remove as much nitrogen as possible from this lower grade natural gas before introducing it into the pipelines. First of all, removal of thenitrogen reduces the volume of gas being conveyed so that only that portion which actually has heating value is sent through the line. Secondly, a gas containing any substantial quantity of nitrogen cannot be employed by'consumers without adjustment of the appliance burners. In other words, the low grade gas could be made interchangeable with the high grade natural gas containing essentially methane.

The present process involves low temperature liquefaction of the feed gas followed by fractional distillation. In prior processes of-this type the gaseous mixture of methane and nitrogen is rectified and the separated nitrogen is compressed, cooled andused as the agent for supplying refrigeration for liquefaction. .In the present invention the major portion of the refrigeration requirements are supplied originally'by the latent heat of vapor- ;ization of a recirculatingstream of liquid methane. The

refrigeration available in the low temperature nitrogen, separated from the cooled feed gas, is used to supply a portion of .the refrigeration requirements. By utilizing a condensing methane cycle to provide refrigeration, a substantial savings in power requirements can be effected, amounting to one-third less than the power required where refrigeration is supplied primarily from the separated nitrogen and a recirculated nitrogen stream. This savings is effected by the reduced quantity of material recirculated and the'lower operating pressures required when methane is used as the refrigerant instead of nitrogen. The con- ,densing methane cycle operates so that each pound of methane provides refrigeration equal to its heat of vaporization plus sensible heat effect, a considerable advantage over the nitrogen cycle where only the sensible heat effect is used for refrigeration.

Briefly, the invention consists first in compressing a portion of the methane product gas (or methane from any suitable source, herein referred to as refrigerant methane) to ,superatmospheric pressure and cooling the compressed refrigerant methane to below its condensation ,temperature by passing it in heat exchange relationship with the product methane being discharged from the frac tionating or distillation column. As the recirculated refrigerant methane is condensed it supplies the heat for vaporization of the product methane and the heat requiretion of the feed gas.

2,823,523 '-Paten,t,ed .E ,b- 1% .958

fiable methane being removed in; liquid form at the bottom liquid at such pressure is separated'from the stream for further expansionat the next step. The major proportion of the expandedrefrigerant'methane stream, which consists of both a-gaseous-and liquid phase, is sent through coils ina heat exchanger to absorb heatfrom and condensethe feed gas (methaneand nitrogen mixture). The

separated liquid is-again expanded toform a mixture of .gas and liquid. Aportion of the liquid phase is advanced to the third and final expansion step, while the remaining gas and liquid mixture is used to subcool the condensed feed gas, or for other cooling purpose. The third expansion step results in gasification of all the liquid to substantially atmospheric pressure. The heat absorbed by the vaporization of zliquid afterthe'third expansion step is preferably employed in condensing the refiuxnitrogen required for thedistillation column.

The invention will-be bettenunderstood by reference to the accompanying drawing, which illustrates diagrammatically the -flow .of materials in accordance with :the present invention.

Referring now vto the drawing, the feed gas which consists of a mixture of methane and nitrogen, the proportion of nitrogen ranging up to, say, 30%,

is either compressed or .expanded.depending upon its initial pressure. The distillation column will be operated at elevated .pressure, say, from ZQOto 300 pounds per square inch. -If the pressure of the feed gas is .higher than the operating pressure of .the .distillationcolumn, then his permitted to expand in the diced expansion turbine '10 from which it is discharged at-the desired pressure. The

expandedfeed gas exits from turbine :10 through-line 12 and enters the feed precooler 16 through line 14. :In

the event the ,feed gas pressure is below that maintained in the distillation column, its pressure must be increased and this is done bypassingtherlow pressure feed through the compressor 18. By-pass 20 is provided around the compressor in the eventthe feed gasis of substantially the same pressure as thedistillation column and requires. no treatment. In compressingthe .feed gas, a certain amountof heat is generated and torernove this heat the gas is passed through after-cooler 22 .which is supplied, with cooling water. Feed gas at a temperature slightly higher than thecooling water temperature and at a .pres sure just in excess of thecolumn operating pressure-them enters the feed precooler 16 which is essentially a heat exchanger. The refrigeration for the preco-ol er is sup plied'by nitrogen discharged from the system and methane product gas, as .will be explained hereafter.

16 the compressed gas flows through the line 24 intothe feed gas condenser 26 where sufficient heat is re- The temperature to moved to condense the feed gas. which the gas must be cooled in order to condense .it depends, of course, upon the pressure and the composi- However, where the distillation column is operating at around 200 pounds per square inch, a 30% nitrogen-70% methanemixture would'be cooled to approximately 230 F. The liquefied gas at this temperature and just in excess of 200 pounds per square inch pressure may be introduced directly into the distillation column through valved line 28, or it may be still further cooled by passing through the liquid feed subcooler 3D. The liquid feed subcooler cools the compressed condensed gas ,to approximately 250- -F.

This subcooled liquid is then introduced into the distillation column, either through the liquid pump 32 or the by-pass 34, depending upon the feed pressure. It is essential, of course, "that the pressure in the line 34 or 36 be in excess of the pressure prevailing within the distillation column 40. In the distillation column the feed is rectified,'the nitrogen being taken 01f as gaseous overhead in line 38, and the methane as liquid product through line 42 into product vaporizer 44. The heat of vaporization of the product methane is utilized to cool the recirculated methane gas which is cycled through the heat exchangers utilized for cooling and condensing the feed stock. As the product methane gains. heat, it is converted back to the gaseous phase and passes from the product vaporizer 44 through the line 46 into the precooler 16 where it absorbs heat from the warm feed gas flowing through the line 14. The product methane gas, now heated to about atmospheric temperature, is compressed to the desired transmission pressure in the product compressor 48, after-cooled in the heat exchanger 51 and sent to storage or into the pipeline, as desired. The heat exchanger 51 is cooled by means of water flowing through the coil disposed therein. V e

To provide the refrigeration required for cooling the feed gas we employ a circulating condensing methane system. Methane comprising part of the product gas, or supplied from any convenient source, is compressed in the compressor 50, shown to the far left of the drawing, and is circulated constantly from the compressor through the system in a closed circuit. This methane may be called refrigerant methane, as distinguished from the product methane. The gas is compressed to a pressure about 10 pounds per square inch greater than the column operating pressure. The compressed gas is after-cooled in the heat exchanger 52 and flows through the line 54 into heat exchanger 56 where it is precooled by gaseous methane which is returning to theicompressors from the feed gas condenser 26. The precooled compressed refrigerant methane gas then flows through the line 58 into the product vaporizer where it is. cooled and partially condensed by the product methane which is being converted back to the gaseous phase in'the vaporizer 44. Further cooling is efiected by passing the compressed refrigerant methane through the coil 6% disposed in the distillation column reboiler 41. The temperature of the liquid methane in the column-reboiler is sufficiently low to condense thecomprcssed and precooled methane which is flowing through coil 60. The liquid methane which has a temperature about F. higher than the column product stream at a pressure about pounds per square inch higher than the column operating pressure then flows through the line 62 and is expanded through the valve 64 to approximzitely onehalf of the column pressure. During expansion the liquid methane is partially converted to the gaseous phase and the two phases flow through line 66 into the high pressure separator 68. In the separator a small portion of the liquid phase is withdrawn from the bottom through line 72 and the major portionof the liquid phaseand allthe gaseousv phase is discharged from the side through line 70. Thetwo phasemethane stream from line 7i) is directed through coils in the feed gas'condenser 26 where it absorbs large quantities of heat from the feed gas flowing couutercurrentthereto as the liquid is vaporized and the gas. warmed. Thewarmed gas is discharged through the line71, passes through heat exchanger 56 where it gives up most of the remainingrefrigeration therein to compressed methaneflowing from the compressor system couin'tereurrent through .the exchanger 5c via line 54, this pointthe methane gas is at substantially atmospheric temperature.. From the .heat

exchanger 56 the' 'ig'aseou-s' stream is directed through the. main line' 57' of- 'the compression system into the ,main'compresso'retl, from whichit is recycled.

The liquid methane withdrawn from the bottom of the separator 68 into the line 72 is then expanded again through valve 73, this time to a medium pressure of approximately one-fourth the column operating pressure and the resulting two phase stream enters the medium pressure separator 76. Again the gas phase and most of the liquid phase are utilized in cooling, being conveyed through the line 78 into the liquid feed subcooler 30, where the liquid is vaporized and the gas warmed, and also through separator coils in the feed gas condenser 26 from which it is directed back to the compressor system through the line 80. This gas also passes through heat exchanger 56 to absorb heat from the freshly compressed methane flowing back to the distillation system through the line 54. Medium pressure methane goes through a first stage compressor 82, an after-cooler 84 and then flows into the main stream 57. It is further compressed in the compressor along with the returned methane from line 72.

Again, the liquid methane withdrawn from the medium pressure separator 76, flows through the line 86 and is expanded through the valve 88 to the final low pressure. The resulting two phase stream is directed to the reflux condenser 90 through line 89. As the liquid is vaporized in passing through the reflux condenser it absorbs heat from the gaseous nitrogen which flows countercurrent thereto,- condensing ,the liquid refluxrequired by the column. The nitrogen stream is introduced at the top of the condenser through the line 38., To utilize all of the remaining refrigeration in the methane gas, which is at the temperature of liquid nitrogen upon being discharged from the reflux condenser, it is directed through the line 92 into the liquid feed subcooler 30 and the feed gas condenser26fromwhich it is conveyed through the line 94 back to the compressor system. The treatment is similar to that of the gaseous methane returning through lines 72 and 80. It passes first through heat exchanger 56 and then into the low pressure compressor 96, through the after-cooler 98, and back into the main stream 57. Thus, it will be seen that all three returning I methane gas streams are joined in the line 57 and subsequently compressed to the desired pressure before being recondensed and recycled through the stepwise expansion system used for supplying the refrigeration for the process.

All of the refrigeration required is not provided directly by the methane condensing cycle just recited. A certain amount of refrigeration is available from the gaseous nitrogen (originally cooled by methane as part of feed gas) being discharged from the top of the distillation column and it is, of course, most economical to utilize the ability of this gas to absorb heat. It is, therefore, passed through the reflux condenser 90 into the separator 98. The temperature of the nitrogen after passing throughthe reflux condenser is sufliciently low so that at least a portion of the nitrogen has been condensed to a liquid. The liquid phase flows from the bottom of the separator through the line back into the distillation column where it is utilized as reflux for the distillation column. The gaseous portion of the nitrogen is discharged from the top of the separator 98 through the line 162 and through liquid feed subcooler .30 to assist the circulating methane in cooling thecondensed feed gas. From the subcooler 30 the gaseous nitrogen flows through the expansion turbine 104 where mechanical energy is recovered and the'temperature and pressure of the nitrogen reduced, and then through the reflux condenser 90 to help cool nitrogen flowing therethrough from line 33. T he partially expanded nitrogen gas then flows through a second turbine 196 via line lttdand after this expansion is circulated once again through the reflux condenser via line 108 at substantially atmospheric pressure. After the/second pass through the I reflux condenser, "the nitrogen flows through line 110 to the subcooler 3t) and condenser 26. The remaining refrigeration in the gas is utilized in the feed precooler 16. The nitrogen flows from the line 112 through the feed precooler 16 and is then vented to the atmosphere or used as a feed stream for chemical manufacture.

It will be apparent to those skilled in the art that variations of the process shown are possible. For example, it is not essential that the liquid methane be expanded in three different steps. It may be expanded only twice to produce one high pressure and one low pressure stream. Furthermore, there are obvious modifications which can be made in the particular form of equipment employed and it is not our intention to limit the invention otherwise than as necessitated by the scope of the appended claims.

The following table summarizes data on the methane refrigeration system in accordance with the invention, setting forth the properties of the methane cooling liquid at various points in the system. The figures are given for column pressures of 200 and 300 pounds per square inch absolute, and for feed to the column through line 28. The medium pressure separator and the liquid feed subcooler are by-passed for the system operating under these conditions.

Summary of data on methane refrigeration system for 100 MMCF. per day of 70% methane-30% nitrogen feed gas Column pressure, p. s. l. a 200 300 Basis, feed gas per..- 1 MOF MOF Reflux Condenser:

Pressure, p. s. i. a.. 22. 5 Inlet temperature, F 261. 5 248. 5 Inlet condition, percent liquid- 75. 9 81. 8 Outlet temperature, F -253 242 Flow rate, b, stream 89-- 6. 10 8. 66 Heat load, B. t. u 1, 159 1, 465 Feed Condenser:

Low pressure methane stream 92, pressure, 1np.s.i.a 13 22.5 et temperature, F 263 242 Outlet tem erature, F 179 --161 Flow rate, b 6.10 8.66 Heat load, B. t. u 152 364 High pressure methane stream 70, pressure,

p. s. a 70 135 Inlet temperature, F 2i7. 5 -199 Inlet condition (corrected for excess v or) percent liquid 67. 5 74. 6 Outlet temperature, F -179 161 Flow rate, lb 28. 90 28.05 Heat load, B. t. u 5, 044 4,195 Total heat load supplied by Methane, B. t. u 5, 196 4, 559 Column Reboiler and Product Vaporizer:

Reboiler heat load, B. t. u 2, 006 2, 177 Vaporizer heat load, 13. t. u 5, 072 4, 439 Recirculating methane pressure, p. s. i. a 210 310 Inlet temperature, gas, F -120 100 Outlet tern erature, liquid, F 176 158 Flow rate, b., stream 58 34. 99 36. 00

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In a process for separating nitrogen from a gaseous mixture of methane and nitrogen by liquefaction and fractional distillation in a distillation column maintained at elevated pressure, the steps of compressing methane refrigerant gas to above said elevated pressure, passing the compressed methane sequentially through a vaporizer containing cold product methane from the distillation column to vaporize the product methane and through cold product methane in the lower reboiler portion of the distillation column to supply heat for operating the column, thereby simultaneously reducing the temperature of the compressed methane to condense said compressed methane, firstly partially expanding said condensed methane to a gas and a liquid phase at about onehalf said elevated pressure, passing the gas phase and a portion of the liquid phase through a heat exchanger to supply part of the refrigeration required to condense the feed gas, secondly further expanding the remainder of the liquid phase to a mixture of gas and liquid at about one-fourth said elevated pressure, passing the gas and most of the liquid at said reduced pressure through a second heat exchanger to supply refrigeration for subcooling said condensed feed gas before introduction into the distillation column, and finally expanding the remaining liquid from said reduced pressure to substantially atmospheric pressure to provide refrigeration for a reflux condenser connected to said distillation column,

2. A process for separating nitrogen from a gaseous mixture of methane and nitrogen feed gas which comprises compressing the feed gas to an elevated pressure and cooling it to below its condensation temperature, rectifying the condensed gas to separate its contained nitrogen and methane, passing the nitrogen resulting from said rectification through a first and second heat exchanger to supply part of the refrigeration for condensing and cooling said feed gas, compressing refrigerant methane from any source to superatmospheric pressure, cooling the compressed methane to below condensation temperature, partially expanding the cooled compressed methane to a gas and a liquid phase, and passing the gas phase through said second heat exchanger to supply a major proportion of the refrigeration required to condense the feed gas, further expanding at least a portion of said liquid phase and passing said further expanded phase through said first exchanger to supply the remainder of the refrigeration required to cool the condensed feed gas.

References Cited in the file of this patent UNITED STATES PATENTS 2,082,189 Twomey June 1, 1937 2,495,549 Roberts Jan. 24,1950 2,500,118 Cooper Mar. 7, 1950 2,534,274 Kniel Dec. 19, 1950 2,534,903 Etienne Dec. 19, 1950 2,583,090 Cost Ian. 22, 1952 2,677,945 Miller May 11, 1954 FOREIGN PATENTS 876,651 France Aug. 10, 1942 

1. IN A PROCESS FOR SEPARATING NITROGEN FROM A GASEOUS MIXTURE OF METHANE AND NITROGEN BY LIQUEFACTION AND FRACTIONAL DISTILLATION IN A DISTILLATION COLUMN MAINTAINED AT ELEVATED PRESSURE, THE STEPS OF COMPRESSING METHANE REFRIGERANT GAS TO ABOVE SAID ELEVATED PRESSURE, PASSING THE COMPRESSED METHANE SEQUENTIALLY THROUGH A VAPORIZER CONTAINING COLD PRODUCT METHANE FROM THE DISTILLATION COLUMN TO VAPORIZE THE PRODUCT METHANE AND THROUGH COLD PRODUCT METHANE IN THE LOWER REBOILER PORTION OF THE DISTILLATION COLUMN TO SUPPLY HEAT FOR OPERATING THE COLUMN, THEREBY SIMULTANEOUSLY REDUCING THE TEMPERATURE OF THE COMPRESSED METHANE TO CONDENSE SAID COMPRESSED METHANE, FIRSTLY PARTIALLY EXPANDING SAID CONDENSED METHANE TO A GAS AND A LIQUID PHASE AT ABOUT ONEHALF SAID ELEVATED PRESSURE, PASSING THE GAS PHASE AND PORTION OF THE LIQUID PHASE THROUGH A HEAT EXCHANGER TO SUPPLY PART OF THE REFRIGERATION REQUIRED TO CONDENSE THE FEED GAS, SECONDLY FURTHER EXPANDING THE REMAINDER OF THE LIQUID PHASE TO A MIXTURE OF GAS AND LIQUID AT ABOUT ONE-FOURTH SAID ELEVATED PRESSURE, PASSING THE GAS AND MOST OF THE LIQUID AT SAID REDUCED PRESSURE THROUGH A SECOND HEAT EXCHANGER TO SUPPLY REFRIGERATION FOR SUBCOOLING SAID CONDENSED FEED GAS BEFORE INTRODUCTION INTO THE DISTILLATION COLUMN, AND FINALLY EXPANDING THE REMAINING LIQUID FROM SAID REDUCED PRESSURE TO SUBSTANTIALLY ATMOSPHERIC PRESSURE TO PROVIDE REFRIGERATION FOR A REFLUX CONDENSER CONNECTED TO SAID DISTILLATION COLUMN. 